Basis of Presentation Basis of Presentation - Narratives (Details) $ in Thousands	Sep. 30, 2018 USD ($) vie	Dec. 31, 2017 USD ($) vie
Variable Interest Entity
Variable interest entities consolidated, count	18	11
Variable interest entities, count	19	12
Variable interest entities unconsolidated, count	1	1
Equity investment in real estate \| $	$ 19,761	$ 20,919
Equity method investments
Variable Interest Entity
Equity investment in real estate \| $	$ 19,800	$ 20,900

The number of variable interest entities held by the company as of the balance sheet date which are consolidated.

No definition available.

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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of variable interest entities held by the company as of the balance sheet date.

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
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Period Type:	instant

- Definition

The number of variable interest entities held by the company as of the balance sheet date which are unconsolidated.

+ References

No definition available.

+ Details

Name:	cpa18_VariableInterestEntitiesUnconsolidatedCount
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Data Type:	xbrli:integerItemType
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Period Type:	instant

- Definition

This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=109237563&loc=d3e33749-111570

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 25
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8813-108599

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Details

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Data Type:	na
Balance Type:
Period Type: